1. Field of the Invention
The present invention relates to a controller for generating jitters in a power converter and a method thereof, and particularly to a controller and a method thereof that can generate jitters in a constant current mode of a power converter.
2. Description of the Prior Art
In a constant current mode of a power converter, a controller for controlling a power switch of a primary side of the power converter includes a peak voltage control unit, a discharge time detection unit, a constant current control unit, and a voltage-controlled oscillator (VCO), which can be applied to operation of the constant current mode of the power converter to provide an approximately predetermined constant current to an output load.
The peak voltage control unit can effectively control a greatest peak current flowing through a secondary side winding of the power converter to be a predetermined value. The discharge time detection unit can generate a discharging signal through a feedback pin and an induced voltage of an auxiliary winding of the power converter, where the discharging signal can be worked as a detection result corresponding to discharge time of the secondary side winding of the power converter. The constant current controller can determine whether charges of the secondary side outputted from a secondary winding during a present switching cycle period is equal to total estimated charges generated from the predetermined constant current according to the discharge time and a switching period provided by a gate pin of the power converter. If variations exist, a control voltage of the VCO is altered so that a clock frequency outputted by VCO is accordingly adjusted, where the adjusted clock frequency influences subsequent switching periods so that total estimated charges of a next switching period are also influenced. Thus, a negative feedback loop is formed. Therefore, the negative feedback loop can make secondary side charges outputted by the secondary side winding during one switching period converge to the total estimated charges, resulting in an average output current of the secondary side winding being approximately converged to the predetermined constant current to achieve a purpose of constant current control.
However, in the constant current mode of the power converter, if jitter voltages of the primary side of the power converter is utilized to generate jitters of the switching period of the power switch, the jitters of the switching period of the power switch may be eliminated by the above mentioned negative feedback loop. Thus, because the jitters of the switching period of the power switch are not generated easily in the constant current mode of the power converter, how to generate the jitters of the switching period of the power switch in the constant current mode of the power converter is an important issue for a designer of the controller of the power switch.